Receiver output control circuit



July 29, 1941. G. MouN'rJoY RECEIVER OUTPUT CONTROL CIRCUIT Filed July 21, 1939 INVENTOR. GARRQRD MOUNTJOY ATTORNEY.

2 QQ DS Patented July 2,9, 1941 UNITED'STTES PATENT OFFICE RECEWER {BUTPUT CONTROL CIRCUIT Garrard Mountjcy, Manhasset, N. Y., assigner to Radio Corporation of America, a corporation of Delaware Application .Fuly 21, 1939, Serial No. 285,695`

(Cl. Z50-20) 4 Claims.

My present` invention relates to modulated carrier receiver output control circuits, and more particularly to an automatic receiver output control circuit adapted to discriminate between speech and dance music in the modulation on the received carrier.

One of the main objects of my present invention is to provide a method of, and means for, substantially preventing reproduction of the received program of a receiver during such periods of the program that are devoted to speech solely.

Another important object of the invention is to provide a gain control circuit for the audio amplifier of a sound transmission system, Wherein the control circuit is capable of automatically discriminating between speech and dance music sequences. e

, Another object of the present invention is to prevent reproduction by a radio receiver during sequences of a dance music radio program devoted solely to speech, and the control of reproduction being responsive to the amplitude of the low audio frequencies of the modulation frequency range.

It can be shown that in the audio output of the detector, when detecting dance music-modulated carrier energy, the amplitude of the audio frequencies up to a frequency oi the order of 150 cycles is far in excess of the amplitude of the same frequencies when receiving speechmodulated carrier energy. This is due to the fact that the drums, bass viols and trombones of the dance music orchestra produce an abundance of low audio notes of the order of 150 cycles. By comparison, ordinary speech lacks these low audio notes. It is then merely necessary to filter off the low audio frequencies up to a frequency of the order of 150 cycles in order to provide a source of control energy that will discriminate between music and speech sequences of a given radio program. It may be stated to be another object of my present invention to provide such a discrimination.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, there is shown a circuit arrangement which embodies the receiver control circuit actuated in response to the intensity ofthe 10W audio frequency portion of the detected audio frequency modulation. The receiving system can be of any conventional type, and for the purposes of this application it is assumed that it is a superheterodyne receiver. Only the detector device l is shown and this may be of the diode type, whose resonant input circuit 2 is tuned to the operating intermediate frequency. The circuit 2 will, of course, be coupled to the output circuit of the prior intermediate frequency amplifier, and those skilled in the art are fully aware of the earlier stages of such a receiver. 'Ihe detector load resistor is designated by the numeral 3, ,theV intermediate frequency bypass condenser 4 being shunted across the load resistor 3. l

The lead 5 is designated by the letters AVC, and it is to be understood that this is the usual automatic volume control connection which is made to the signal grids of the prior transmission tubes. The purpose of the automatic volume control connection, as is well known, is to maintain the carrier amplitude at the input circuit 2 substantially uniform regardless of a wide carrier amplitude variation at the signal collector of the receiver. The audio modulation developed across resistor 3 is applied to the control grid of an audio frequency amplifier 6, and the audio coupling path is shown as embodying a slidable tap 1 and an audio coupling condenser 8.

The tap 'I functions as a manual volume con-V trol device, since adjustment thereof determines the intensity of the audio voltage impressed at the control grid of audio tube 6. The plate circuit of the audio amplifier tube 6 includes the primary Winding of the audio transformer 9, and it is to be understood that the latter may be coupled to one or more audio frequency ampli lier stages'followedby any well known type of reproducer, such as a loudspeaker. The cathode of audio amplifier 6 is established at ground potential, while the control grid of the tube is connect-ed to ground through a path which includes the resistor I0,;variable resistor Il and the negative potential source l2. In the absence of direct current voltage developed across resistor li the control grid of tube 6 has applied thereto a negative bias from source I2 which permits tube 6 to operate with normal audio frequency amplification. In other Words, the negative voltage applied from source l2 is such that the audio amplier 6 operates as a conventional audio amplifier with desired gain. The resistor j Il has developed thereacross a direct current voltage which depends upon the control circuit which is now to b-e described.

The audio gain control circuit is automatic in operation, and depends essentially upon the amplitude of the W audio frequency content of the audio modulation voltage developed across resistor 3. In order to provide a predetermined amplitude of the required low audio frequencies, there is provided a low pass iilter network I3 whose input lead Ill is connected to the slidable tap 1. The low pass filter is constructed, in general, with a pair of series resistors I5 and I5 and a pair of shunt condensers I6 and I6. Each of the series resistors may have a magnitude, for example, of one megohm, while each of the shunt condensers may have a magnitude of 0.902 mf. It is to be understood that the constants of the low pass filter network i3 are chosen so that the filter passes the low audio frequencies up to a frequency of the order of 150 cycles, and greatly attenuates the audio frequencies above that range of low audio frequencies. Those skilled in the art are fully acquainted with the manner of constructing such a low pass audio filter and, therefore, it is not believed necessary for the purpose of this application to go into further detail on the manner of constructing the network I3. The audio frequency Voltage outputof network I3 is impressed upon the input electrodes of an audio frequency amplifier I?, and the amplified audio'voltage developed across resistor IS, located in the plate circuit of tube l1, is impressedrupon the cathode of a diode recti-er I 9, I

The cathode 2G of rectifier I9 is connected to the plate endof resistor I3 through an audio coupling condenser 2|, while the anode 22 of diode I9 is connected to the negative terminal of current source Eil through the load resistor 23. The resistor 24.is connected between cathode 2i) and the negative terminal of source 5i? in order to provide a direct current path for the rectified audio currents of diode IS. The condenser25, shunted acrossA load resistor 23, bypasses the low audio frequencies, and the direct current voltage developed across resistor is applied tothe Ycontrol grid of control tube through a lead 3l. The plate of tube 3B isV connected to the grid, or negative, end of resistor II through a resistor rid, 4while the cathode of tube .3e is connected to the negative terminal of the direct current source 5I). .The positive terminal of current source EIlAisY connected to the positive end of resistor II, and it is to be understood that the arrow shown passing through resistor I I denotes that the resistor is variable in magnitude..

To explain the operation of the present invention, let it first be assumed that the receiver has been` tuned to receive a station which is broadcasting speech in the form, for example, of an announcer-explanation l which occurs between dance music periods. In such case the amplitude of the low audio frequencies below a frequency of the order of 150 cycles will be extremely small. In fact, audio frequencies below 150 cycles will be substantially'absent from the modulation on the carrier in such case. Accordingly, the amplifier I'I will have a minimum voltage developed across resistor i8, and, consequently, the direct current voltage'developed across diode load resistor 23 will besubstantially zero.' Itis to be noted that the amplitude of the direct current voltage developed 'across resistor 23 bears a direct relation to the amplitude of the audio voltage output of, filter network I3. It necessarily follows'that the space current flowing between the cathode and plate of control tube 30 is at a maximum since no negative voltage is applied to the grid of tube 3i) and, therefore, there will be developed across resistor II maximum direct current voltage.

The direct current voltage developed across resistor il is applied to the control grid of audio amplifier 6 in the same polarity sense as is the bias from source I2. The magnitude of resistor II is to be adjusted to a value such that the control grid of tube Ei will be biased substantially to cutoii when the station being received has speech modulation on the carrier. Of course, it is to be understood that the magnitude of resistor I i can be adjusted so that the gain of tube 6 will be greatly reduced, instead of being out ofi, when speech signals are being received.

If, now, the received station is broadcasting dance music, or, in general, is modulating the carrier with a large proportion of the low audio frequencies, the audio voltage developedat the output of network I3 will increase. In rthe case of dance music the lowaudio frequencies below a frequency of the order of cycles are present in substantially highamplitude. Accordingly, the amplified audio voltage developed across resistor I3 increases, with the result thatthe direct current voltage developed across resistor 23 increases. This, in turn, results in an increase in the negative bias on the control grid of control tube The current flow of the control tube is sharply reduced, and the gain reduction voltage developed across resistor Il is thereby eliminated. Hence, when the received carrier has dance music modulation thereon, which meansv tl the low audio frequencies of the order of 150 cycles are present in increased intensity, thegain of the audio amplifier 6 will be restored to normal.

It will now be seen that the control tube 30-is actuated by the low audio frequencies lof the modulation on the received carrier. The effect of using the present audio gain control circuit is to have the receiver solely reproduce dance music. During speech periods there will exist a hiatus in reproduction. Of course, if it is desired to listen to the speech sequences of a dance music program, it is onlynecessary to adjust resistor II so that substantially zero voltage is produced across the resistor when vthe bias applied to the grid of tube 3i# is a minimum. The receiver will now operate in normal manner.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A method of reception which includes demodulating received sound-modulated carrier waves, filtering from the sound modulation frequencies resulting from demodulation a band of low audio frequencies up to a frequency of the order of 150 cycles, the amplitude of the frequency components in said filtered band being for dance music-modulated carrier waves greatly in` excess of the amplitude of the same components when receiving speech-modulated carrier waves, amplifying the demodulated energy, and sub-- stantially inhibiting said amplification in response to a decrease of solely the amplitude of the components of said band below a predeterminedy intensity level representative of speech modulation.

2. In a system for amplifying and reproducing sound-representative waves, means for separating from the waves components of low audio frequency and lying in a band up to substantially 150 cycles, said components having a substantially greater amplitude in the case of said sound being dance music rather than speech, means for deriving a direct current voltage from the said components Whose magnitude varies with solely the amplitude of the components, and means for utilizing the said voltage to reduce amplification of the said waves in response to said voltage magnitude decreasing below a predetermined level representative of speech.

3. In combination with the audio amplifier of a radio receiver, means including a low pass filter, adapted to pass a band of audio frequencies up to a frequency of the order of 150 cycles, upon whose input is impressed the audio input of said for normally preventing operation of said amplifier, and auxiliary means, responsive to solely the relatively high amplitude of the very low audio frequencies up to substantially 150 cycles present in music Voltage, for disabling said operation-prevention means.

GARRARD MOUNTJ OY. 

